I've had helpful 'popups' on mobile sites that I couldn't close because the close button was perpetually out of screen, as they were not designed for the small screen. To my surprise, I've seen this on a number of big sites.

It baffles me that nobody ever noticed this bug, but on the other hand I can see how this would happen. The developers probably never noticed because they already had the cookie that prevents the popup from showing. Still...